An object of the present invention is to provide a cured product which is suitable as a material for a layer of low refractive index in a multilayer diffractive optical element, can exhibit a desired chromatic aberration reducing effect over a near-infrared wavelength region to a shortwave infrared wavelength region in a case of being used in the multilayer diffractive optical element, and can exhibit high transmittance in this wavelength range; a curable resin composition suitable for obtaining this cured product; and a diffractive optical element and a multilayer diffractive optical element including this cured product. Provided are a curable resin composition which includes oxide nanoparticles including indium and cerium, a monofunctional or higher functional (meth)acrylate compound, and a dispersant; a cured product formed of the curable resin composition; a diffractive optical element and a multilayer diffractive optical element; and oxide nanoparticles used in this composition.